1. Field of the Invention
The present invention relates to an electronic device employing an organic semiconductor. More particularly, it relates to a photoelectronic device such as a photoelectric conversion element and an EL element.
2. Description of the Related Art
Compared to inorganic compounds, organic compounds include more varied material systems, and through appropriate molecular design it is possible to synthesize organic materials having various functionalities. Further, the organic compound is characterized in that films and the like formed using the organic compound demonstrate great pliancy, and superior processability can also be achieved by polymerization. In light of these advantages, in recent years, attention has been given to photonics and electronics employing functional organic materials.
Photonic techniques which make use of photophysical qualities of organic compounds have already played an important role in contemporary industrial techniques. For example, photosensitive materials, such as a photoresist, have become indispensable in a photolithography technology used for fine processing of semiconductors. In addition, since the organic compounds themselves have properties of light absorption and concomitant light emission (fluorescence or phosphorescence), they have considerable applicability as light emitting materials such as laser pigments and the like.
On the other hand, since organic compounds do not have carriers themselves, they essentially have superior insulation properties. Therefore, in the field of electronics where the electrical properties of organic materials are utilized, the main conventional use of organic compounds is insulators, where organic compounds are used as insulating materials, protective materials and covering materials.
However, there are means for making massive amounts of electrical current flow in the organic materials which is essentially insulators, and they are starting to be put to practical use in the electronics field. The “means” discussed here can be broadly divided into two categories.
The first of these, represented by conductive polymers, is means in which a π-conjugate system organic compound is doped with an acceptor (electron acceptor) or a donor (electron donor) to give the π-conjugate system organic compound a carrier (Reference 1: Hideki Shirakawa, Edwin J. Louis, Alan G. MacDiarmid, Chwan K. Chiang, and Alan J. Heeger, “Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetyrene, (CH)x”, Chem. Comm., 1977, 16, 578-580). By increasing the doping amount, the carrier will increase up to a certain area. Therefore, its dark conductivity will also increase together with this, so that significant electricity will be made to flow.
Since the amount of the electrical flow can reach the level of a normal semiconductor or more, a group of materials which exhibit this behavior can be referred to as organic semiconductors (or, in some cases, organic conductors).
This means of doping the acceptor/donor to improve the dark conductivity to make the electrical current flow in the organic material is already being applied in part of the electronics field. Examples thereof include a rechargeable storage battery using polyaniline or polyacene and an electric field condenser using polypyrrole.
The other means for making massive electrical current flow in the organic material uses an SCLC (Space Charge Limited Current). The SCLC is an electrical current which is made to flow by injecting a space charge from the outside and moving it, the current density of which is expressed by Child's Law, i.e., Formula 1, shown below. In the formula, J denotes a current density, ∈ denotes a relative dielectric constant, ∈0 denotes a vacuum dielectric constant, μ denotes a carrier mobility, V denotes a voltage, and d denotes a distance (hereinafter, referred to as “thickness”) between electrodes applied with the voltage V:J=9/8·∈∈0μ·V2/d3  Formula 1
Note that the SCLC is expressed by Formula 1 in which no carrier trap when the SCLC flows is assumed at all. The electric current limited by the carrier trap is referred to as a TCLC (Trap Charge Limited Current), and it is proportionate to a power of the voltage, but both the SCLC and the TCLC are currents that are subject to bulk limitations. Therefore, both the SCLC and the TCLC are dealt with in the same way hereinbelow.
Here, for comparison, Formula 2 is shown as a formula expressing the current density when an Ohm current flows according to Ohm's Law. σ denotes a conductivity, and E denotes an electric field strength:J=σE=σ·V/d  Formula 2
In Formula 2, since the conductivity σ is expressed as σ=neμ (where n denotes a carrier density, and e denotes an electric charge), the carrier density is included in the factors governing the amount of the electrical current that flows. Therefore, in an organic material having a certain degree of carrier mobility, as long as the material's carrier density is not increased by doping as described above, the Ohm current will not flow in a material which normally does not have few carriers.
However, as is seen in Formula 1, the factors which determine the SCLC are the dielectric constant, the carrier mobility, the voltage, and the thickness. The carrier density is irrelevant. In other words, even in the case of an organic material insulator with no carrier, by making the thickness d sufficiently small, and by selecting a material with a significant carrier mobility μ, it becomes possible to inject a carrier from the outside to make the current flow.
Even when this means is used, the current flow amount can reach the level of a normal semiconductor or more. Thus, an organic material with a great carrier mobility μ, in other words, an organic material capable of latently transporting a carrier, can be called an “organic semiconductor”.
Incidentally, even among organic semiconductor elements which use the SCLC as described above, organic electroluminescent elements (hereinafter, referred to as “organic EL elements”) which use both the photonic and electrical qualities of functional organic material as photoelectronic devices, have particularly demonstrated remarkable advancement in recent years.
The most basic structure of the organic EL element was reported by W. Tang, et al. in 1987 (Reference 2: C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987)). The element reported in Reference 2 is a type of diode element in which electrodes sandwich an organic thin film having a total thickness of approximately 100 nm and being constituted by laminating a hole-transporting organic compound and an electron-transporting organic compound, and the element uses a light emitting material (fluorescent material) as the electron-transporting compound. By applying voltage to the element, light-emission can be achieved as from a light emitting diode.
The light-emission mechanism is considered to work as follows. That is, by applying the voltage to the organic thin film sandwiched by the electrodes, the hole and the electron injected from the electrodes are recombined inside the organic thin film to form an excited molecule (hereinafter, referred to as a “molecular exciton”), and light is emitted when this molecular exciton returns to its base state.
Note that, types of molecular excitons formed by the organic compound can include a singlet excited state and a triplet excited state, and the base state is normally the singlet state. Therefore, emitted light from the singlet excited state is referred to as fluorescent light, and the emitted light from the triplet excited state is referred to as phosphorescent light. The discussion in this specification covers cases of contribution to the emitted light from both of the excited states.
In the case of the organic EL element described above, the organic thin film is normally formed as a thin film having a thickness of about 100 to 200 nm. Further, since the organic EL element is a self-luminous element in which light is emitted from the organic thin film itself, there is no need for such a back light as used in a conventional liquid crystal display. Therefore, the organic EL element has a great advantage in that it can be manufactured to be extremely thin and lightweight.
Further, in the thin film having a thickness of about 100 to 200 nm, for example, the time from when the carrier is injected to when the recombination occurs is approximately several tens of nanoseconds, given the carrier mobility exhibited by the organic thin film. Even when the time required by for the process form the recombination of the carrier to the emission of the light, it is less than an order of microseconds before the light emission. Therefore, one characteristic of the organic thin film is that response time thereof is extremely fast.
Because of the above-mentioned properties of thinness and lightweightness, the quick response time, and the like, the organic EL element is receiving attention as a next generation flat panel display element. Further, since it is self-luminous and its visible range is broad, its visibility is relatively good and it is considered effective as an element used in display screens of portable devices.
Further, in addition to the organic EL element, an organic solar battery is another representative example of an organic semiconductor element using organic material (i.e., an organic semiconductor) capable of transporting carriers latently, which is to say having a certain degree of carrier mobility.
In short, the organic solar battery utilizes an opposite structure to the organic EL element. That is, its structure is similar to the most basic structure of the organic EL element, where the organic thin film having the two-layer structure is sandwiched by electrodes (Reference 3: C. W. Tang, “Two-layer organic photovoltaic cell”, Applied Physics Letters, vol. 48, No. 2, 183-185 (1986)). A photoelectric current generated by causing light to be absorbed into the organic thin film is used to obtain an electromotive force. The electric current that flows at this time can be understood as follows: the carrier generated by the light flows due to the carrier mobility present in the organic material.
In this way, the organic material, which was considered as having no purpose in the electronics field other than its original purpose as an insulator, can be made to perform central functionalities in various electronic devices and photoelectronic devices by skillfully devising the organic semiconductor. Accordingly, research in organic semiconductors has become robust at present.
Description has been made above regarding two methods using the organic semiconductor as means for flowing the electric current to the organic material which is essentially an insulator. However, each of these two methods has a different problem.
First, in the case where the acceptor and the donor are doped to the organic semiconductor to increase the carrier densities, the conductivity is actually improved but the organic semiconductor itself loses its own physical properties (light absorption, phosphorescence, etc.) which it originally had. For example, when a phosphorescent-light emitting π-conjugate system polymer material is doped with the acceptor/donor, its conductivity increases but it stops emitting light. Therefore, in exchange for obtaining the functionality of conductivity, the other various functionalities which the organic material possesses are sacrificed.
Further, although there is an advantage in that various conductivities can be achieved by adjusting a doping amount of the acceptor or the donor, no matter how much acceptor and donor are doped to increase the carrier, it is difficult to constantly obtain a carrier density equivalent to a metal or of an inorganic compound that is equivalent to a metal (e.g., nitride titan or other such inorganic compound conductor). In other words, with respect to conductivity, it is extremely difficult to surpass an inorganic material, except for in several examples. Thus, the only remaining advantage is that the organic material is extremely workable and pliant.
On the other hand, in the case where the SCLC (hereinafter, SCLC includes a photoelectric current) is made to flow to the organic semiconductor, the physical properties that the organic semiconductor originally had are not lost. A representative example of such is none other than the organic EL element, in which the light emission from the fluorescent material (or phosphorescent material) is utilized even when the electric current is made to flow. The organic solar battery also utilizes the functionality of light absorption by the organic semiconductor.
However, as can be understood by looking at Formula 1, since the SCLC is inversely proportionate to the 3rd power of the thickness d, the SCLC can only be made to flow through a structure consisting of electrodes sandwiched to both surfaces of extremely thin films. More specifically, in light of the general carrier mobility of organic materials, the structure must be an ultra thin film of approximately 100 nm to 200 nm.
It is true, however, that by adopting the above-mentioned ultra thin film structure, a significant amount of SCLC can be made to flow at low voltage. One reason why the organic EL element such as the one discussed in Reference 2 is successful is because the thickness of its organic thin film is designed as a uniformly ultra thin film having a thickness of approximately 100 nm.
However, the fact that the thickness d must be made extremely thin actually becomes the biggest problem when the SCLC is made to flow. First, in the 100 nm thin film, it is easy for pinholes and other such defects to develop, and short circuits and other such problems occur due to these, causing a concern that yield may deteriorate. Further, not only does the mechanical strength of the thin film decline, but also the manufacturing process itself is restricted because the film must be an ultra thin film.
Further, when the SCLC is used as the electric current, the physical properties that the organic semiconductor itself originally possessed are not lost, and there is an advantage in that various functionalities can be produced. However, deterioration of the functionality of the organic semiconductor is accelerated by making the SCLC flow. For example, looking at the organic EL element as an example, it is known that the lifetime of the element (i.e., the half-life of the brightness level of the emitted light) deteriorates almost in inverse proportion to its original brightness, or, in other words, to the amount of electrical current that is made to flow (Reference 4: Yoshiharu SATO, “The Japan Society of Applied Physics/Organic Molecular Electronics and Bioelectronics”, vol. 11, No. 1 (2000), 86-99).
As described above, in the device where the acceptor or the donor is doped to produce conductivity, functionalities other than the conductivity are lost. Further, in the device where the SCLC is used to produce the conductivity, the flowing of massive amounts of an electrical current through the ultra thin film becomes a cause of problems regarding the element's reliability and the like.
Incidentally, in photoelectronic devices using the organic semiconductors, such as organic EL elements and organic solar batteries, there is also a problem with respect to efficiency.
The organic EL element will be discussed as an example. The light emitting mechanism of the organic EL element is that the injected hole and electron recombine with each other to be converted into light. Therefore, theoretically, it is possible to extract at most one photon from the recombination of one hole and one electron, and it is not be possible to extract a plurality of photons. That is, the internal quantum efficiency (the number of emitted photons with respect injected carriers) should be at most 1.
However, in reality, it is difficult to bring the internal quantum efficiency close to 1. For example, in the case of the organic EL element using the fluorescent material as the light emitting body, the statistical ratio of generation for the singlet excited state (S*) and the triplet excited state (T*) is considered to be S*:T*=1:3 (Reference 5: Tetsuo TSUTSUI, “Textbook of the 3rd seminar at Division of Organic Molecular Electronics and Bioelectronics, The Japan Society of Applied Physics”, p. 31 (1993)). Therefore, the theoretical limit of the internal quantum efficiency is 0.25. Furthermore, as long as the fluorescent quantum yield from the fluorescent material is not φf, the internal quantum efficiency will drop even lower than 0.25.
In recent years, attempts have been made to use phosphorescent materials to use the light emission from the triplet excited state to bring the internal quantum efficiency's theoretical limit close to 0.75 to 1, and the efficiency actually surpassing that of fluorescent material has been achieved. However, in order to achieve this, it is necessary to use a phosphorescent material with a high phosphorescent quantum efficiency φp. Therefore, the range of selection for the material is unavoidably restricted. This is because organic compounds that can emit phosphorescent light at room temperature are extremely rare.
In other words, if means could be structured for improving the electrical current efficiency (the brightness level generated in relation to the electrical current) of the organic EL element, this would be a great innovation. If the electrical current efficiency is improved, a greater level of brightness can be produced with a smaller electrical current. Conversely, since the electrical current can be reduced for achieving a certain brightness level, the deterioration caused by the massive amount of electrical current made to flow to the ultra thin film as described above can be reduced.
The inverse structure of the organic EL element, which is to say the photoelectric conversion such as in the organic solar battery, is inefficient at present. As described above, in the organic solar battery using the conventional organic semiconductor, the electrical current does not flow if the ultra thin film is not used. Therefore, electromotive force is not produced, either. However, when the ultra thin film is adopted, a problem arises in that the light absorption efficiency is poor (i.e., the light cannot be completely absorbed). This problem is considered to be the largest reason for the poor efficiency.
In light of the foregoing discussion, the electronic device using the organic semiconductor has a shortcoming in that when the massive electrical current is made to flow in a device utilizing the physical properties that are unique to the organic material, the reliability and yield from the device is influenced unfavorably. Furthermore, particularly in the photoelectronic device, the efficiency of the device is poor. These problems basically can be said to arise from the “ultra thin film” structure of the conventional organic semiconductor element.